Gingival epithelial cells heterozygous for Toll-like receptor 4 polymorphisms Asp299Gly and Thr399ile are hypo-responsive to Porphyromonas gingivalis

Porphyromonas gingivalis induce apoptosis in human gingival epithelial cells through a gingipain-dependent mechanism

BACKGROUND

The oral pathogen Porphyromonas gingivalis has been shown to modulate apoptosis in different cell types, but its effect on epithelial cells remains unclear.

RESULTS

We demonstrate that primary human gingival epithelial cells (HGECs) challenged with live P. gingivalis for 24 hours exhibit apoptosis, and we characterize this by M30 epitope detection, caspase-3 activity, DNA fragmentation and Annexin-V staining. Live bacteria strongly upregulated intrinsic and extrinsic apoptotic pathways. Pro-apoptotic molecules such as caspase-3, -8, -9, Bid and Bax were upregulated after 24 hours. The anti-apoptotic Bcl-2 was also upregulated, but this was not sufficient to ensure cell survival. The main P. gingivalis proteases arginine and lysine gingipains are necessary and sufficient to induce host cell apoptosis. Thus, live P. gingivalis can invoke gingival epithelial cell apoptosis in a time and dose dependent manner with significant apoptosis occurring between 12 and 24 hours of challenge via a gingipain-dependent mechanism.

CONCLUSION

The present study provides evidence that live, but not heat-killed, P. gingivalis can induce apoptosis after 24 hours of challenge in primary human gingival epithelial cells. Either arginine or lysine gingipains are necessary and sufficient factors in P. gingivalis elicited apoptosis.

TLR4 as a risk factor for periodontal disease: a reappraisal

AIM

To determine whether genetic variants of the TLR4 gene are associated with either chronic or aggressive periodontitis.

METHODS

A systematic electronic search of literature was conducted to identify all published studies without any language restriction on the association between TLR4 and periodontal diseases, including chronic periodontitis and aggressive periodontitis. All case-control studies evaluating the TLR4 Asp299Gly and Thr399Ile polymorphisms in chronic or aggressive periodontitis were identified. A meta-analysis of the studies that fulfilled the inclusion criteria was performed.

RESULTS

Seven studies comprising 744 chronic periodontitis cases and 855 controls and four studies consisting of a total of 295 aggressive periodontitis cases and 456 controls were included in the meta-analysis. In the pooled analysis, the TLR4 299Gly allele (TLR4+896 A>G) appeared to be a genetic risk factor for susceptibility to chronic periodontitis with a random effects and fixed effects odds ratio (OR) of 1.43 [95% confidence interval (CI):1.04-1.97; p=0.03]. On the other hand, the TLR4 399Ile polymorphism (TLR4+1196 C>T) showed a protective effect against aggressive periodontitis with a random effects OR of 0.29 (95% CI: 0.13-0.61; p=0.001).

CONCLUSION

Our results suggest that the alleles 299Gly and 399Ile in TLR4 can be a potential genetic marker for periodontal disease.

Immune interventions of human diseases through toll-like receptors

Toll-like receptors (TLRs) are the immune sensors for infections, triggering robust innate immune activation followed by protective adaptive immunity against various infectious diseases. Recent evidence, however, has suggested that TLRs are involved in the pathogenesis of many diseases, including not only infectious diseases but also autoimmune diseases, allergy and atherosclerosis. Therefore, prophylactic or therapeutic application of TLR-based immune interventions should be potent, but their safety must be demonstrated using experimental animal models as well as human resources, including analysis of single nucleotide polymorphisms. Here, we focus on recent advances in understanding of the protective and pathogenic roles of TLRs in human diseases.

Immunohistochemical localization of Toll-like receptors 1-10 in periodontitis

BACKGROUND/AIM

In periodontitis, bacteria and pathogen-associated molecular patterns are sensed by Toll-like receptors (TLRs), which initiate intracellular signaling cascades that may lead to host inflammation. In this study, the expression and distribution of TLRs (TLR-1 to TLR-10) were immunohistochemically detected in gingival epithelium and connective tissue.

METHODS

Immunohistochemistry was used for the localization of TLRs in gingival tissue samples from 10 patients with chronic periodontitis and 10 healthy controls; these samples were obtained during routine periodontal flap operations and during extraction operations performed for retained wisdom teeth, respectively. For the evaluation, epithelial layers were stratified to basal, spinous, and superficial layers, and the percentages of TLR-positive cells were determined.

RESULTS

Both healthy and periodontitis gingival tissues expressed all TLRs except TLR-10. In patients with periodontitis, epithelial cells showed increased TLR expression towards the basal layer. Healthy controls showed more variable cellular TLR expression and distribution between the layers of epithelium. In the connective tissue, consistently higher TLR expression was found within the periodontitis group compared to the healthy group.

CONCLUSIONS

For the first time, the cellular expression and distribution of TLR-1 to TLR-10 have been studied in periodontitis, indicating that TLR-1 to TLR-9 are differentially expressed both in connective tissue and epithelial layers. Except for TLR-7 and TLR-8, all the other TLRs showed statistically significant differences between patients with periodontitis and healthy controls, suggesting their involvement in the pathogenesis of periodontitis.

Streptococcus cristatus attenuates Fusobacterium nucleatum-induced interleukin-8 expression in oral epithelial cells

BACKGROUND AND OBJECTIVE

Oral epithelial cells may be invaded by a polymicrobial intracellular flora, including pathogens together with commensals. Various oral pathogens can induce the production of interleukin-8, a potent neutrophil chemotractant, in oral epithelial cells. Evidence from the gut suggests that commensal species may modulate inflammatory responses to pathogens. The aim of this study was to examine the interleukin-8 responses of oral epithelial cells to an oral pro-inflammatory species, Fusobacterium nucleatum, in combination with an oral commensal, Streptococcus cristatus.

MATERIAL AND METHODS

KB, TERT-2, TR146 and SCC15 cells were cocultured with F. nucleatum and S. cristatus, either alone or in combination, at 37 degrees C in 5% CO2 under various conditions. The mRNA expression of interleukin-8 was analyzed by reverse transcription-polymerase chain reaction and protein secretion was measured by enzyme-linked immunosorbent assay.

RESULTS

F. nucleatum alone evoked a potent interleukin-8 response, whereas S. cristatus alone did not induce significant interleukin-8 expression in oral epithelial cells. When present together, S. cristatus attenuated the F. nucleatum-induced interleukin-8 production in the four oral epithelial cell lines to varying degrees. The inhibitory effect of S. cristatus was independent of its viability and its co-aggregation with F. nucleatum, was not related to soluble bacterial products and appeared to require bacterial contact with epithelial cells. Similar effects were seen with several other species of oral streptococci.

CONCLUSION

Our data suggest that S. cristatus may exert immunomodulatory effects on the interleukin-8 response of oral epithelial cells to F. nucleatum challenge.

TLR4 and IL-18 gene variants in aggressive periodontitis

AIM

We aimed to assess the association of different genotypes with increased aggressive periodontitis susceptibility by studying functional relevant variants in the pathogen-recognition receptor Toll-like receptor 4 (TLR4) and variants in the promoter region of the pro-inflammatory cytokine interleukin-18 (IL-18).

MATERIAL AND METHODS

One hundred and eleven patients with aggressive periodontitis and 80 periodontally healthy controls were genotyped for four functional variants in the TLR4 gene (c.896A>G and c.1196C>T) and in the IL-18 promoter (c.-368G>C and c.-838C>A). The genotype and allele frequencies, as well as the frequency of combined genotypes were compared between study groups.

RESULTS

There were no statistical differences in genotype and allele frequencies within the four variants between the groups. All study subjects were further classified into carriers and non-carriers of at least one variant of both genes. The logistic regression analysis adjusted for gender and smoking showed no association between carrier status of at least one variant of both genes and periodontal status (OR=1.41, 95% CI: 0.43-4.70).

CONCLUSIONS

Our results reject the hypothesis that functionally relevant IL-18 and TLR4 gene mutations have a major effect on aggressive periodontitis susceptibility alone or in combination.

Functional consequences of toll-like receptor 4 polymorphisms

Toll-like receptor 4 (TLR4) is an important pathogen recognition receptor that recognizes mainly lipopolysaccharide (LPS) of Gram-negative bacteria, but also structures from fungal and mycobacterial pathogens, as well as endogenous ligands. Two nonsynonymous polymorphisms of TLR4, Asp299Gly and Thr399Ile, have been suggested to alter the function of the receptor. Some, but not all, studies have proposed that these polymorphisms lead to reduced cytokine response and increased susceptibility to Gram-negative infections. In this review, we compare studies that assessed the effect of the Asp299Gly and Thr399Ile polymorphisms on susceptibility to Gram-negative infections and examine the phenotypic consequences of these polymorphisms. In addition, we review the geographical distribution of TLR4 polymorphisms and present a model for evolutionary pressures on the TLR4 genetic make-up.

TLR4 and S1P receptors cooperate to enhance inflammatory cytokine production in human gingival epithelial cells

Toll-like receptors (TLR) are pattern recognition receptors for highly conserved microbial molecular patterns. Activation of TLR is a pivotal step in the initiation of innate, inflammatory, and immune defense mechanisms. Recent findings indicate that G protein-coupled receptors (GPCR) may modulate TLR signaling, but it is unclear which GPCR are involved in this process. One such cooperation between GPCR and TLR can be attributed to the sphingosine 1-phosphate (S1P) receptor family. The S1P receptors (S1P1-5) are a family of GPCR with a high affinity for S1P, a serum-borne bioactive lipid associated with diverse biological activities such as inflammation and healing. In this study, we show that pro-inflammatory cytokine production, including IL-6 and IL-8, was increased with LPS and concomitant S1P stimulation. Furthermore, elevated cytokine production following LPS and S1P challenge in human gingival epithelial cells (HGEC) was significantly reduced when TLR4, S1P1 or S1P3 signaling was blocked. Our study also shows that S1P1 and S1P3 expression was induced by LPS in HGEC, and this elevated expression enhanced the influence of S1P in its cooperation with TLR4 to increase cytokine production. This cooperation between TLR4 and S1P1 or S1P3 demonstrates that TLR4 and GPCR can interact to enhance cytokine production in epithelial cells.

Beyond good and evil in the oral cavity: insights into host-microbe relationships derived from transcriptional profiling of gingival cells

In many instances, the encounter between host and microbial cells, through a long-standing evolutionary association, can be a balanced interaction whereby both cell types co-exist and inflict a minimal degree of harm on each other. In the oral cavity, despite the presence of large numbers of diverse organisms, health is the most frequent status. Disease will ensue only when the host-microbe balance is disrupted on a cellular and molecular level. With the advent of microarrays, it is now possible to monitor the responses of host cells to bacterial challenge on a global scale. However, microarray data are known to be inherently noisy, which is caused in part by their great sensitivity. Hence, we will address several important general considerations required to maximize the significance of microarray analysis in depicting relevant host-microbe interactions faithfully. Several advantages and limitations of microarray analysis that may have a direct impact on the significance of array data are highlighted and discussed. Further, this review revisits and contextualizes recent transcriptional profiles that were originally generated for the specific study of intricate cellular interactions between gingival cells and 4 important plaque micro-organisms. To our knowledge, this is the first report that systematically investigates the cellular responses of a cell line to challenge by 4 different micro-organisms. Of particular relevance to the oral cavity, the model bacteria span the entire spectrum of documented pathogenic potential, from commensal to opportunistic to overtly pathogenic. These studies provide a molecular basis for the complex and dynamic interaction between the oral microflora and its host, which may lead, ultimately, to the development of novel, rational, and practical therapeutic, prophylactic, and diagnostic applications.

Role of Porphyromonas gingivalis SerB in gingival epithelial cell cytoskeletal remodeling and cytokine production

The SerB protein of Porphyromonas gingivalis is a HAD family serine phosphatase that plays a critical role in entry and survival of the organism in gingival epithelial cells. SerB is secreted by P. gingivalis upon contact with epithelial cells. Here it is shown by microarray analysis that SerB impacts the transcriptional profile of gingival epithelial cells, with pathways involving the actin cytoskeleton and cytokine production among those significantly overpopulated with differentially regulated genes. Consistent with the transcriptional profile, a SerB mutant of P. gingivalis exhibited defective remodeling of actin in epithelial cells. Interaction between gingival epithelial cells and isolated SerB protein resulted in actin rearrangement and an increase in the F/G actin ratio. SerB protein was also required for P. gingivalis to antagonize interleukin-8 accumulation following stimulation of epithelial cells with Fusobacterium nucleatum. SerB is thus capable of modulating host cell signal transduction that impacts the actin cytoskeleton and cytokine production.

Sphingosine 1-phosphate 1 and TLR4 mediate IFN-beta expression in human gingival epithelial cells

IFN-beta production is a critical step in human innate immune responses and is primarily controlled at the transcription level by highly ordered mechanisms. IFN-beta can be induced by pattern-recognition receptors such as the TLR4. S1P1 is a G protein-coupled receptor, which has a high affinity for sphingosine 1-phosphate (S1P). Although many of the receptors and signaling pathways leading to the expression of IFN-beta have been identified and characterized, it is still unclear how IFN-beta is regulated in primary human gingival epithelial cells (HGECs). In this study, we demonstrate that S1P1 and TLR4, acting in unison, play an important role in IFN-beta expression at the protein and mRNA level in HGECs. We demonstrate that the expression of both IFN-beta and IFN-inducible protein-10 (CXCL-10) is significantly up-regulated by LPS and S1P or LPS and a specific S1P1 agonist. This enhanced innate immune response is attenuated in HGECs by small interfering RNA knockdown of either TLR4 or S1P1. Moreover, we show that triggering of TLR4 results in the increased expression of S1P1 receptors. Furthermore, we found that IFN-regulatory factor 3 activation was maximized by LPS and S1P through PI3K. Our data show that triggering TLR4 increases S1P1, such that both TLR4 and S1P1 acting through PI3K enhancement of IFN-regulatory factor 3 activation increase IFN-beta expression in epithelial cells. The functional association between TLR4 and the S1P1 receptor demonstrates a novel mechanism in the regulation of IFN-beta and CXCL-10 in human primary gingival epithelial cells.

Interleukin-1beta modulates proinflammatory cytokine production in human epithelial cells

Periodontitis is a chronic human inflammatory disease initiated and sustained by dental plaque microorganisms. A major contributing pathogen is Porphyromonas gingivalis, a gram-negative bacterium recognized by Toll-like receptor 2 (TLR2) and TLR4, which are expressed by human gingival epithelial cells (HGECs). However, it is still unclear how these cells respond to P. gingivalis and initiate inflammatory and immune responses. We have reported previously that HGECs produce a wide range of proinflammatory cytokines, including interleukin-6 (IL-6), IL-8, granulocyte-macrophage colony-stimulating factor, tumor necrosis factor alpha (TNF-alpha), and IL-1beta. In this study, we show that IL-1beta has a special role in the modulation of other inflammatory cytokines in HGECs challenged with P. gingivalis. Our results show that the increased production of IL-1beta correlates with the cell surface expression of TLR4, and more specifically, TLR4-normal HGECs produce fourfold more IL-1beta than do TLR4-deficient HGECs after challenge. Moreover, blocking the IL-1beta receptor greatly reduces the production of "secondary" proinflammatory cytokines such as IL-8 or IL-6. Our data indicate that the induction of IL-1beta plays an important role in mediating the release of other proinflammatory cytokines from primary human epithelial cells following challenge with P. gingivalis, and this process may be an inflammatory enhancement mechanism adopted by epithelial cells.

Differential regulation of innate immune response genes in gingival epithelial cells stimulated with Aggregatibacter actinomycetemcomitans

BACKGROUND AND OBJECTIVE

The gingival epithelium provides the first line of defense against colonization by periodontal pathogens, both as a physical barrier and by the production of inducible innate immune mediators such as beta-defensins and pro-inflammatory cytokines. The gram-negative bacterium Aggregatibacter actinomycetemcomitans is implicated in the pathogenesis of localized aggressive periodontitis, although the bacterium is found widely in the healthy population. We hypothesized that gingival epithelial cell-derived innate immune mediators triggered in response to A. actinomycetemcomitans infection may play an important role in increased susceptibility to infection.

MATERIAL AND METHODS

Primary cultures of human gingival epithelial cells were cultured in the presence of A. actinomycetemcomitans. Total mRNA was examined for the presence of innate immune markers using RT-PCR.

RESULTS

We show here that the mRNA levels of human beta-defensin 2 and interleukin-8 are elevated by live cultures of a clinical isolate of A. actinomycetemcomitans in cultured gingival epithelial cells from healthy individuals, but not by A. actinomycetemcomitans lipopolysaccharide. Cells from a patient with localized aggressive periodontitis, however, did not respond to this bacterial stimulation. In contrast, the pro-inflammatory cytokine interleukin-19 was induced in cells from both localized aggressive periodontitis and healthy subjects. Examination of Toll-like receptors and associated adapter molecules indicated lower levels of Toll-like receptor 2 mRNA in the localized aggressive periodontitis patient-derived cells compared with cells from healthy subjects.

CONCLUSION

These results suggest that a differential expression of innate immune response genes to A. actinomycetemcomitans in the gingival epithelium could be an underlying factor of susceptibility to localized aggressive periodontitis.

Polymorphisms in Toll-like receptors-2 and -4 are not associated with disease manifestations in acute Q fever

Coxiella burnetii is a macrophage-tropic, Gram-negative organism, which causes acute Q fever infection in humans. This zoonotic infection causes illness ranging from asymptomatic seroconversion to severe and protracted disease featuring hepatitis and pneumonia. Interactions between C. burnetii lipopolysaccharide (LPS) and host Toll-like receptors (TLR)-2 and -4 have been implicated in pathogen recognition, phagocytosis and signaling responses. Nonconservative single nucleotide polymorphisms in the coding regions of TLR-2 (Arg677Trp and Arg753Gln) and TLR-4 (Asp299Gly) have been found to correlate with mycobacterial infections and Gram-negative sepsis respectively. Associations between the TLR-2 and -4 polymorphisms, illness characteristics and immune response parameters were examined in subjects with acute Q fever (n=85) and comparison subjects with viral infections (n=162). No correlation was demonstrated between these polymorphisms and susceptibility to Q fever, illness severity or illness course.

Antibacterials as anti-inflammatory agents: dual action agents for oral health

BACKGROUND

Inflammatory processes with a range of specialized cells and biochemical mediators form a complex network of inter-related signal transducing pathways that relay information to preserve normal functions. Advances in molecular analyses of the information relay pathways for their constituents and principal ligands along with mechanisms utilized by the host for microbial recognition have stimulated interest in therapeutic agents with dual functionalities i.e. antibacterial and anti-inflammatory effects.

AIM

This review examines clinically tested agents for oral health applications with both antimicrobial and anti-inflammatory effects to include antibiotics, antimicrobials and phenolics.

RESULTS

Bis-phenols such as triclosan, representing a unique dual functional therapeutic for routine oral hygiene, with its demonstrated clinical effects on inhibiting the dental plaque biofilm, reducing inflammation (gingivitis) and subsequent periodontitis is described. Cyclines, comprising another class of approved anti-inflammatory agents used at the patient level for oral health is discussed. Dual active agents in current clinical practice for systemic conditions are highlighted to summarize the clinical validity of dual function agents as an emerging therapeutic strategy.

CONCLUSIONS

Clinical studies demonstrate therapeutic benefits of agents with dual functionality with their effects on microorganisms and the concomitant host inflammatory response. Advances in microbial pathogenesis and resultant inflammation will facilitate progress in this emerging area poised to be a significant milestone for dental therapeutics.

Innate immunogenetics: a tool for exploring new frontiers of host defence

The discovery of innate immune genes, such as those encoding Toll-like receptors (TLRs), nucleotide-binding oligomerisation domain-like receptors (NLRs), and related signal-transducing molecules, has led to a substantial improvement of our understanding of innate immunity. Recent immunogenetic studies have associated polymorphisms of the genes encoding TLRs, NLRs, and key signal-transducing molecules, such as interleukin-1 receptor-associated kinase 4 (IRAK4), with increased susceptibility to, or outcome of, infectious diseases. With the availability of high-throughput genotyping techniques, it is becoming increasingly evident that analyses of genetic polymorphisms of innate immune genes will further improve our knowledge of the host antimicrobial defence response and help in identifying individuals who are at increased risk of life-threatening infections. This is likely to open new perspectives for the development of diagnostic, predictive, and preventive management strategies to combat infectious diseases.

Activation of protective responses in oral epithelial cells by Fusobacterium nucleatum and human beta-defensin-2

Oral epithelia are constantly exposed to non-pathogenic (commensal) bacteria, but generally remain healthy and uninflamed. Fusobacterium nucleatum, an oral commensal bacterium, strongly induces human beta-defensin-2 (hBD2), an antimicrobial and immunomodulatory peptide, in gingival epithelial cells (GECs). hBD2 is also expressed in normal oral tissue leading to the hypothesis that oral epithelia are in an activated state with respect to innate immune responses under normal in vivo conditions. In order to test this hypothesis, global gene expression was evaluated in GECs in response to stimulation by an F. nucleatum cell wall (FnCW) preparation and to hBD2 peptide. FnCW treatment altered 829 genes, while hBD2 altered 209 genes (P<0.005, ANOVA). Many induced genes were associated with the gene ontology categories of immune responses and defence responses. Consistent with the hypothesis, similar responses were activated by commensal bacteria and hBD2. These responses included up-regulation of common antimicrobial effectors and chemokines, and down-regulation of proliferation markers. In addition, FnCW up-regulated multiple protease inhibitors, and suppressed NF-kappaB function and the ubiquitin/proteasome system. These global changes may protect the tissue from inflammatory damage. Both FnCW and hBD2 also up-regulated genes that may enhance the epithelial barrier. The findings suggest that both commensal bacteria and hBD2 activate protective responses of GECs and play an important role in immune modulation in the oral cavity.

Lack of association between chronic periodontitis and the Toll-like receptor 4 gene polymorphisms in a Czech population

BACKGROUND AND OBJECTIVE

Periodontitis is a bacterially induced chronic inflammatory disease and a major cause of tooth loss among adults. Toll-like receptors are signal molecules essential for the cellular response to bacterial cell wall components. The aim of this study was to investigate relationships between chronic periodontitis and variations in the TLR4 gene.

MATERIAL AND METHODS

A total of 171 patients with chronic periodontitis and 218 unrelated controls were genotyped for the Asp299Gly (896A>G) and Thr399Ile (1196C>T) polymorphisms of the TLR4 gene.

RESULTS

Both variants were in nearly complete linkage disequilibrium. No homozygotes for the less common alleles, 299Gly and 399Thr, respectively, were found. The prevalence of the Asp299Gly and the Thr399Ile heterozygotes was 5.3% and 5.0% in controls, and 7.0% and 7.0% in periodontitis patients.

CONCLUSION

In conclusion, TLR4 gene polymorphisms were not significantly associated with the susceptibility to, or severity of, chronic periodontitis in our population.

Differential activation of NF-kappaB and gene expression in oral epithelial cells by periodontal pathogens

To investigate the molecular effects of the periodontopathogens Fusobacterium nucleatum (FN) and Porphyromonas gingivalis (PG) on the oral epithelium, the H400 oral epithelial cell line was cultured in the presence of non-viable bacteria. Following confirmation of the presence of transcripts for the bacterial pattern recognition receptors in H400 cells, Toll-like receptors -2, -4 and -9, and components of the NF-kappaB signalling pathway, immunocytochemical analyses were performed showing that NF-kappaB was activated within 1 h of exposure to both periodontopathogens. A significantly greater number of NF-kappaB nuclear translocations were apparent following H400 cell exposure to FN as compared with PG. Gene expression analyses indicated that transcripts known to be regulated by the NF-kappaB pathway, including cytokines/chemokines TNF-alpha, IL-1beta, IL-8, MCP-1/CCL2 and GM-CSF, were up-regulated following 4 and 24 h of exposure to both periodontopathogens. In addition, H400 periodontopathogen exposure resulted in differential regulation of transcripts for several cytokeratin gene family members. Consistent with the immunocytochemical data, microarray results indicated that FN induced a greater number of gene expression changes than PG following 24 h of exposure, 609 and 409 genes, respectively. Ninety-one genes were commonly differentially expressed by both periodontopathogens and represented biological processes commonly associated with periodontitis. Gene expression analyses by reserve transcriptase-polymerase chain reaction (RT-PCR) of molecules identified from the microarray data sets, including Heme oxygenase-1, lysyl oxidase, SOD2, CCL20 and calprotectin components, confirmed their differential expression profiles induced by the two periodontopathogens. FN and PG have clearly different molecular effects on oral epithelial cells, potentially highlighting the importance of the composition of the plaque biofilm in periodontitis pathogenesis.

Advancement of the relationship between gene polymorphisms of Toll-like receptors and infectious diseases

Toll-like receptors (TLRs) were identified as transmembrane signal transduction proteins in recent years. As a group of pattern recognition receptors (PRRs), TLRs play important roles in the innate immunity by recognizing pathogen-associated molecular patterns (PAMPs). PAMPs trigger TLR signaling cascades, leading to the release of proinflammatory cytokines, and play critical roles in infectious diseases. Genetic polymorphisms affect the susceptibility of individuals to infectious diseases, and single nucleotide polymorphisms of TLR genes are associated with the inflammatory responses and susceptibility to infectious diseases. Most studies have indicated that the Asp299Gly and Thr399Ile of TLR4 as well as the Arg753Gln and Arg667Trp of TLR2 had close correlations with the pathogenesis of infectious diseases. Other genetic polymorphisms of TLRs have also been reported. In this review, we summarized and discussed the function of TLR family and the relationship between genetic polymorphisms of TLRs and infectious diseases.

Polymorphisms of TLR4 but not CD14 are associated with a decreased risk of aggressive periodontitis

OBJECTIVE

To study whether there is an association between the frequency of functional polymorphisms in the toll-like receptor 4 (TLR4) and cluster differentiation 14 (CD14) genes and periodontitis.

METHODOLOGY

Genotyping for the TLR4 single-nucleotide polymorphisms (SNPs) Asp299Gly, Thr399Ile and the CD14 SNPs -159 and -1359 was completed for subjects with periodontal disease compared with control subjects. Two disease populations were investigated: 73 subjects with aggressive periodontitis (AgP; 28 males, 45 females) and 95 males with chronic periodontitis (CP). The TLR4 and CD14 polymorphisms were determined using SNaPshot primer extension with capillary electrophoresis. Comparison of allele and genotype frequencies for each polymorphism was by Fisher's exact test or chi2 analysis.

RESULTS

The TLR4 Asp299Gly genotype was present in a significantly (p=0.026) lower proportion of AgP subjects (5.5%) compared with control subjects (16.3%). The unadjusted odds ratio for the Asp299Gly genotype to be associated with AgP was 0.30, 95% confidence interval 0.10-0.91. No differences were found in the prevalence of the TLR4 Asp299Gly genotype in men with CP (18.9%) compared with an age-matched control group with no evidence of periodontitis (17%). In addition, there was no difference in the distribution of the CD14 polymorphisms in either the AgP or CP populations studied compared with controls.

CONCLUSION

It is concluded that in West European Caucasians, the Asp299Gly TLR4 gene polymorphism is associated with a decreased risk of AgP but not CP. Promoter polymorphisms of the CD14 gene, however, did not influence susceptibility to inflammatory periodontitis in the population cohorts studied.

Differential activation of human gingival epithelial cells and monocytes by Porphyromonas gingivalis fimbriae

Humans develop periodontitis in response to challenge by microbial dental plaque. Inflammation begins after perturbation of gingival epithelial cells by subgingival bacteria interacting through pattern-recognition receptors, including the Toll-like receptors (TLR). Porphyromonas gingivalis is a major periodontopathogen that interacts with epithelial cells through its cell surface fimbriae (FimA), leading to colonization and/or invasion. Previous work by our group has established membrane CD14 as an essential coreceptor for TLR2-mediated activation of transfected cell lines by P. gingivalis FimA. We have shown that gingival epithelial cells express TLR2 but not CD14 on their cell surfaces. We thus speculated that P. gingivalis FimA does not readily activate epithelial innate immune responses but rather functions to promote P. gingivalis colonization in the absence of a vigorous FimA-induced response. This hypothesis was verified by the findings that primary human gingival epithelial cells responded poorly to FimA in terms of interleukin (IL)-6, IL-8, granulocyte-macrophage colony-stimulating factor, and tumor necrosis factor alpha responses, in stark contrast to the marked response to other TLR2 agonists (Pam3Cys, FSL-1) that are not strictly dependent on CD14. On the other hand, CD14-expressing human primary monocytes responded with high levels of the same cytokines to both FimA and the control TLR2 agonists. The gingival epithelial cells failed to respond to FimA even in the presence of exogenously added soluble CD14. These data indicate that the gingival epithelial cell hyporesponsiveness to FimA is attributable to the lack of membrane-expressed but not soluble CD14. In conclusion, P. gingivalis FimA differentially activates human monocytes and epithelial cells, perhaps reflecting different tactics used by P. gingivalis when interacting with different host cell types or a host strategy to limit inflammation.